Plasma-chemical reactors and processes seek to use plasma to initiate desirable chemical reactions. Plasma can be used to promote chemical reactions in liquids and gases, and on the surfaces of solids. Present commercial plasma systems are used for printing, for treating water and for sterilizing surfaces, for example. An impediment to the wider adoption of commercial plasma-chemical reactors is the scale and expense of conventional atmospheric pressure plasma technology. The cost, size, weight, and high voltages characteristic of typical plasma-chemical reactors limit greatly the potential of conventional plasma reactor technology. Ozone treatment is a particularly attractive application of plasma-chemical technology but the cost, size, and weight of most existing systems render them of limited value for many commercial uses.
Existing systems include electron-beam systems and discharge-based systems, such as the pulsed corona or dielectric barrier discharge (DBD) reactors. Electron beam-based systems require the creation of free electrons and their acceleration to high energies under vacuum conditions. This process typically requires high voltages (tens to hundreds of kV). The electrons are introduced into a gas reactor chamber to bombard one gas or a mixture of gases. This can produce fragmentation (dissociation) of the molecules from the gas or the gas mixture. The reaction chamber requires a robust entry point for the electron beam because this point (often a metal foil through which the electrons pass) is subjected to pressures and heat generated in the reaction vessel. High voltages of one hundred (or more) kV are generally required to accelerate the electrons so that they are able to enter the chamber. The requirements for high voltage and vacuum equipment raise the cost and complexity of these systems to a level that limits the utility of the systems.
Discharge based systems create high energy electrons directly within the treated gas volume via application of locally intense electric fields. Such plasma-chemical reactors include dielectric barrier and corona discharge systems. The discharge systems often operate at pressures of hundreds of Torr to beyond 1 atmosphere and require voltages of at least several kV and typically more than 10 kV. Both dielectric barrier systems and corona discharge systems tend to produce inhomogeneous plasmas that are characterized by streamers. The reactor volumes in these systems tend to be large (milliliters to hundreds of liters), which restricts the influence of the reactor wall on the plasma chemistry.
Ozone can be produced when oxygen (O2) molecules are dissociated by an energy source into oxygen atoms. Collisions of free oxygen atoms with oxygen molecules produce ozone (O3), which must be generated at the point of treatment because the lifetime of O3 in air at atmospheric pressure is on the order of minutes. Ozone is the strongest oxidant and disinfectant available commercially. Mechanisms of disinfection using ozone include direct oxidation/destruction of bacterial cell walls, reactions with radical by-products of ozone decomposition, and damage to the constituents of nucleic acids. Particular commercially available DBD systems for the large scale production of ozone for municipal water treatment, for example, are large (as long as 10-15 ft. in length) and have demanding power requirements (150-200 kVA). Furthermore, the conversion of feedstock gases into O3 is typically inefficient. Existing commercial processes for producing O3 in large volume typically convert 15%-18% of the oxygen (O2) feedstock gas into O3. This low efficiency for the conversion of feedstock gas to ozone is a result of the fact that ozone is produced only within, or in the vicinity of, the streamers produced in air or oxygen by DBD systems. Maintenance of such systems is also problematic owing to a large number of ceramic parts and fouling of device components by nitric acid. Existing dielectric barrier discharge technology is also sensitive to the level of organic impurities in the oxygen feedstock gas.
There are additional drawbacks to existing commercial plasma-chemical devices and systems. Dielectric barrier discharge structures are most commonly used in present day commercial systems. These uncomplicated devices apply high voltages to electrodes separated by a dielectric (often, glass or quartz). Typical macroscopic reactors rely upon microdischarge streamers that are nominally 100 μm in diameter and statistically distributed in space and time. Conversion efficiencies are low which, therefore, requires large volumes of oxygen (or air) flows to generate reasonable amounts of O3. Moisture and organic contaminants in the feedstock gas are another problem with conventional ozone generating systems because the system can be fouled and rendered less efficient, or disabled, as a result of nitric acid build up on the reactor wall or on vacuum fittings.
Research by some of the present inventors and colleagues at the University of Illinois has resulted in new microcavity and microchannel plasma device structures, as well as new applications. A particularly promising class of microcavity plasma device arrays is formed in metal and metal oxide. Large-scale, low-cost arrays of microplasma devices that can be flexible are formed by inexpensive processes with metal electrodes encapsulated in metal oxide.
One previous application and publication by several of the present inventors and colleagues has described the production of ozone in microchannels. Specifically, Park et al. U.S. Pat. No. 8,442,091 discloses microchannel lasers having a microplasma gain medium. In that patent, microplasma acts as a gain medium with the electrodes sustaining the plasma in the microchannel. Reflectors can be used in conjunction with the microchannel for obtaining optical feedback and lasing in the microplasma medium in devices of the invention for a wide range of atomic and molecular species. Several atomic and molecular gain media will produce sufficiently high gain coefficients that reflectors (mirrors) are not necessary. FIG. 4 of that patent also discloses a microchemical reactor that is suitable for air purification and ozone production because of the channel lengths and large power loadings (watts deposited per unit volume) of the plasma that are available. However, fabrication costs associated with channels of extended length present an obstacle to commercialization of this technology for applications that would benefit from ozone production.
Some of the present inventors and colleagues have developed devices that produce high quality plasmas in microchannels. For example, linear arrays of 25-200 μm wide channels have been fabricated in glass by replica molding and micropowder blasting and have been demonstrated to be capable of generating low temperature, nonequilibrium microplasmas. See, Sung, Hwang, Park and Eden, “Interchannel optical coupling within arrays of linear microplasmas generated in 25-200 μm wide glass channels,” Appl. Phys. Lett. 97, 231502 (2010). Parallel microchannels have also been fabricated in nanostructured alumina (Al2O3) via a nanopowder blasting process, and shown to provide the capability for routing, and controlling the flow of, packets of low temperature, nonequilibrium plasma. See, Cho, Park and Eden, “Propagation and decay of low temperature plasma packets in arrays of dielectric microchannels,” Appl. Phys. Lett. 101, 253508 (2012). Further development and research on these and additional microchannel structures by some of the present inventors and colleagues produced ozone microreactors capable of generating ozone and fragmenting gas molecules. See, [0062]-[0066] of commonly owned Eden et al., US Published Patent Application 2013/0071297, published Mar. 21, 2013. The ozone microreactor in the '297 Application included 12 microchannels that supported a flow rate of 0.5 standard liters per minute and ozone generation efficiencies exceeding 150 g/kWh.
Current technology for ozone production generally differs with the scale of ozone production required. Reactors tend to be custom-designed for particular applications. As an example, commercial reactors for ozone production for high throughput applications in municipal water treatment and pulp processing employ technology and system designs that differ considerably from those of lower production rate units. Commercial installations are often custom designed, difficult to scale, require large amounts of power, and are generally inefficient as well as sensitive to contaminants in the feedstock gas flow stream.
Several manufacturers currently offer reactors designed for kilogram/hour ozone production rates that are typical of many municipal water treatment facilities. Typical reactors include a number of cylindrical DBD plasma tubes, each of which is separately fused. Plasma produced in each tube is spatially inhomogeneous and ozone production occurs predominately in the vicinity of the streamers. On the other hand, smaller scale applications requiring lower throughput (e.g., <100 g/hr) often employ corona reactors.
A portable ozone generator is described in U.S. Pat. No. 7,157,721 (“'721 patent”). In the '721 device, both sides of a glass or ceramic plate are coated with conductive materials to form electrodes having different areas. Such a device produces a corona discharge in the region lying outside the smaller of the two electrodes. An ozone device based upon this corona discharge mixes ozone with water in flow channels that are formed in plastic. No microchannels exist in the ozone-producing reactor. Another manufacturer provides a modular approach to ozone generation that is based upon corona discharge cells. However, because the corona discharge reactors are not flat, nor is the plasma confined to microscopic channels, these reactors are not readily or easily combined and, in particular, are not amenable to being stacked. Furthermore, the voltages required of corona discharge systems are high (multi-kV) and conversion efficiencies (oxygen or air→ozone) are low.